1. Field of the Invention
The present invention relates to image reading apparatus, and more particularly to an image reading apparatus using light emitting diodes (LEDs) as the light source for document reading and to a control method thereof.
2. Description of the Related Art
As light sources for high-speed image reading apparatus, halogen lamps, fluorescent lamps and xenon tubes have been employed. These light sources, however, have various problems such that they emit a significant amount of heat, that they consume a significant amount of electric power, that they are large and heavy, and that they need time to stabilize light intensity. As a solution to such problems, the white light emitting diode (LED) array is known where two or more white LEDs of low heat generation and low power consumption are arranged in an array.
The white LED array, however, different from halogen lamps, fluorescent lamps and xenon tubes, have a problem that when some LEDs in the array fail it is not easy to spot the faulty LEDs visually.
If some LEDs in the array fail, light intensity may decrease locally. For example, if a certain number of LEDs have failed in one of plural LED blocks constituting the LED array, or if a certain number of LEDs have failed in two neighboring LED blocks, light intensity becomes low locally. Then there will be quality fluctuations in the acquired image data. If an image is printed on paper by a printer based on such image data, image quality becomes uneven in the recorded image and the user may be disappointed.